1. Technical Field
The invention relates to pneumatic tires and in particular to pneumatic tires having tie bars extending circumferentially between adjacent tread blocks in a pair of circumferentially extending ribs located on opposite sides of a mid circumferential plane of the tire. More particularly, the tie bars are tapered in a first direction in one of the ribs and in an opposite direction in the other of said ribs to make changes in RAT (residual aligning torque) without changing the carcass or road contacting tread pattern of the tire.
2. Background Information
Pneumatic tires such as used for passenger and light trucks, have a tread pattern which extends circumferentially about the tire. The tread consists of a plurality of circumferentially and laterally extending grooves which divide the tread into generally circumferentially extending ribs formed by a plurality of either continuous or discontinuous tread blocks. The tread blocks may be separated by lateral grooves or slots which provide for expelling water to prevent hydroplaning and provide better traction in snow and mud. Even when these ribs are symmetrical and equally spaced on opposite sides of a mid-circumferential plane of the tire, they can produce a moment on the tread at the contact patch with the roadway causing a twisting moment on the tire. This moment provides a tendency for the vehicle to drift left or right when the steering wheel is released. This moment or force at the contact patch is referred to as the residual aligning torque (RAT). The effect of this twisting moment and a detailed discussion relating to the various factors thereof, and prior attempts to reduce or eliminate the same, is discussed in detail in U.S. Pat. No. 5,944,082, the contents of which are incorporated herein by reference. The tire construction and method set forth in U.S. Pat. No. 5,944,082 provides draft angles in lateral grooves of certain ribs, which angles are sloped forward on one side of the mid-circumferential plane and sloped backward on the other side of the plane. These sloped grooves allegedly result in the tread blocks sloping forward and backward during forward motion of the vehicle which induces a torque to compensate for the inherent residual aligning torque due to the tread pattern.
The present invention provides a pneumatic tire having a plurality of tie bars extending circumferentially between adjacent tread blocks in selected circumferentially extending ribs located on opposite sides of a mid-circumferential plane of the tire wherein the tie bars have sloped top surfaces extending in a forward direction in one of the ribs and in a reverse rotational direction in the corresponding axial spaced rib.
Another feature of the invention is the ability to make changes in the residual aligning torque (RAT) without changing the carcass or road contacting tread pattern by utilizing tapered tie bars extending in the fore and aft directions in opposing ribs of the tire tread pattern.
Still another feature of the invention is forming the tie bars with various heights and tapers to achieve a desired aligning moment and effect on the RAT, and in which the width of the tie bar can also be varied to effect this change in RAT.
Another feature of the invention is to provide a pneumatic tire having tapered tie bars in which the tie bars are molded integrally with the tread blocks of the tire without requiring substantial changes in the molding process or materially increasing the cost of the tire.
Another aspect of the tapered tie bars is the ability to form the taper in either slots and/or grooves which extend in a generally lateral direction with respect to the mid-circumferential plane of the tire, either perpendicularly thereto or at an angle, in order to achieve various changes in the RAT depending upon the particular tread pattern of the tire thereby avoiding redesigning the particular tread pattern.
A still further feature of the invention is to provide the tapered tie bars with either a full or partial width with respect to the lateral length of the groove and/or slot, and with either a full height or reduced height before the start of the taper with respect to the adjacent tread block, and in which the taper can extend completely to the base of the adjacent tread block or to a selected height of the wall of the adjacent tread block.
Another aspect of the invention is that modification of existing molds is made possible by removing material therefrom enabling a cost effective solution to changing a large inventory of molds at a fraction of the cost of replacing the molds.
A further aspect of the invention is to provide a change in RAT in a tire having an asymmetrical tread pattern by providing tapered tie bars between adjacent circumferentially spaced lugs on opposite sides of a geometric centerline of the tire tread, wherein the angle of taper and spacing from the centerline need not be equal and opposite, but will differ depending upon the particular asymmetrical pattern of the tire tread.
The foregoing advantages, construction and operation of the present invention will become more readily apparent from the following description and the accompanying drawings.